Among elderly psychiatric patients hyponatremia may be caused by many factors including co-morbid medical conditions, psychogenetic polydipsia & drugs. Elderly psychiatric patients appear to develop hyponatremia more often than younger patients. This increased prevalence of hyponatremia in elderly depressed patients appears to be especially prominent among those individuals treated w/specific serotonin reuptake inhibito (SSRIs). The mechanism of the hyponatremia in this population is not yet understood. We hypothesized that SSRIs may induce an abnormality in secretion of ADH.